Species-specific differences in the susceptibility of fungi towards the antifungal protein AFP depend on C3 saturation of glycosylceramides
AFP is an antimicrobial peptide (AMP) produced by the filamentous fungus Aspergillus giganteus and a very potent inhibitor of fungal growth without affecting the viability of bacteria, plant or mammalian cells. It targets chitin synthesis and causes plasma membrane permeabilization in many human and plant pathogenic fungi, but its exact mode of action is not known. We have recently proposed adoption of the \"damage-response framework of microbial pathogenesis\" put forward by Pirofksi and Casadevall in 1999 regarding the analysis of interactions between AMPs and microorganisms, thus, predicting that the cytotoxic capacity of a given AMP is relative and depends not only on the presence/absence of its target(s) in the host and the AMP concentration applied but also on other variables, such as microbial survival strategies. We show here using the examples of three filamentous fungi (Aspergillus niger, Aspergillus fumigatus, Fusarium graminearum) and two yeasts (Saccharomyces cerevisiae, Pichia pastoris) that the important parameters defining the AFP susceptibilities of these fungi are (i) the presence/absence of glycosylceramides, (ii) the presence/absence of {Delta}3(E)-desaturation of the fatty acid chain therein, and (iii) the (dis)ability of these fungi to respond to AFP inhibitory effects with the fortification of their cell walls via increased chitin and {beta}-(1,3)-glucan synthesis. These observations support the adoption of the damage-response framework to holistically understand the outcome of AFP inhibitory effects.\n\nImportanceOur data suggest a fundamental role of glycosylceramides in the susceptibility of fungi towards AFP. We discovered that only a minor structural difference in these molecules - the saturation level of their fatty acid chain, controlled by a 2-hydroxy fatty N-acyl-{Delta}3(E)-desaturase - is a key to understanding the inhibitory activity of AFP. As glycosylceramides are important components of fungal plasma membranes, we propose a model which links AFP-mediated inhibition of chitin synthesis in fungi with its potential to disturb plasma membrane integrity.